The invention relates to a method for rolling a tube to a smaller diameter, in which at least one area of the outside of the tube is brought into contact with at least one roll that is rotatable around its axis, whereby said roll and the tube are brought into a relative movement with respect to another. The contact surface of the roll and the tube, considered between two planes spaced in the longitudinal direction of the tube, lies at an angle, to the longitudinal axis of the tube.
A method of this type is known from the U.S. Pat. No. 3,354,682, with which one produces a truncated pyramid shaped tube by means of an internal, truncated pyramid shaped mandrel placed in the cylindrical starting tube and two eccentric rolls, of which the rotation axis is perpendicular to the longitudinal direction of the tube. The roll surfaces comprise a triangular groove which delimit together the square cross section. In addition to being eccentric, the rolls also comprise an interrupted roll surface, so that the total roll length is limited to the effective developed length of the circumference of the roll.
This has the objection, that for tubes of different size and diameter, other rolls and other mandrels are required which takes much labour, makes a large investment necessary in rolls and mandrels, and makes an economic operation difficult.